An in-depth examination of the Fenian underground in Canada during the 1860s, using primary sources to examine such issues as ethno-religious conflict, the relationship between Catholicism and Irish nationalism, and the efforts of the secret police force to infiltrate and undermine the Fenian Brotherhood. Special attention is paid to the ways in which the state responded to the threat posed by Irish revolutionaries who supported an Irish-American invasion of Canada as a means to hit back at the British Empire and pave the way for Irish independence.